1. Field of the Invention
The present invention relates to a liquid-immersion exposure method and a liquid-immersion exposure apparatus, which are used for performing exposure of a substrate through a projection optical system and a liquid as a result of immersing a portion between a surface of the substrate and a final lens surface of the projection optical system into the liquid.
2. Description of the Related Art
Projection exposure apparatus which forms a circuit pattern on a reticle (mask) onto, for example, a substrate by exposure using a projection optical system is well known. In recent years, there is an increasing demand for an economical exposure apparatus having high resolution. In such a situation, liquid-immersion exposure is drawing attention as a method for meeting the demand for high resolution. In liquid-immersion exposure, the numerical aperture (NA) of the projection optical system is increased as a result of using a liquid for a medium at a substrate-side of the projection optical system. When the refractive index of the medium is n, the NA of the projection optical system is represented by NA=n·sin θ. Therefore, when the medium has a refractive index that is higher than that of air (n>1), the NA can be increased to n. Accordingly, in liquid-immersion exposure, when the NA is increased, resolution R of the exposure apparatus, which is represented by R=k1(λ/NA) (where k1 is a process constant and λ is the wavelength of a light source), is consequentially made smaller (that is, the resolution is consequentially increased).
A local fill method has been proposed (described in PCT Domestic Re-Publication Patent Publication No. 99/49504). In the local fill method, a light-path space between a surface of a substrate and a final lens surface of a projection optical system is locally filled with a liquid in liquid-immersion exposure. However, since, in the local fill method, the liquid is circulated in the narrow gap between the surface of the substrate and the final lens surface of the projection optical system, the supply and recovery of the liquid may not be properly carried out. That is, the liquid is not properly supplied and recovered when a stage is moved. This may give rise to the following phenomena:(1) The liquid cannot be completely held below the final lens, causing it to fly to the surroundings. Therefore, the liquid remains on, for example, a measuring sensor, provided at the stage, or on the substrate; and (2) A film interface of the liquid becomes unstable, causing air bubbles to mix with the liquid.
Accordingly, in (1), when the liquid remains on the substrate, an exposure failure may occur due to process-related problems. On the other hand, when the liquid remains on the measuring sensor, exposure precision may be reduced due to an error in measurement. In (2), the characteristics of the projection optical system are impaired, thereby further reducing the exposure precision. This may result in exposure failure.
To overcome such problems, a method of confining the liquid by surrounding with gas the liquid between the surface of the substrate and the final lens surface of the projection optical system has been proposed (described in Japanese Unexamined Patent Application Publication No. 2004-289126). According to this method, the flying of the liquid to the surroundings, occurring when the liquid cannot be completely held between the surface of the substrate and the final lens of the projection optical system, can be minimized. However, regardless of where the structure discussed in Japanese Unexamined Patent Application Publication No. 2004-289126 is used, when the substrate is moved through a predetermined distance at a speed that is greater than or equal to a certain speed, the liquid flies to the surroundings from the space below the final lens. In addition, when the speed is further increased, the liquid remains on the substrate or the measuring sensor. That is, when the stage is moved at a speed that is greater than or equal to a certain speed, the liquid is left on the substrate or on the measuring sensor at the stage. A maximum movement speed of the stage at which the liquid does not remain in the surroundings as a result of flying from the space below the final lens is called “limit speed.” The limit speed depends upon the distance of movement of the substrate, and is known to have a tendency to decrease as the movement distance is increased. Further, it is known that, during the movement, it depends upon the liquid repellency of a surface which the liquid contacts, and that the limit speed tends to decrease as the liquid repellency is decreased. That is, the limit speed is a parameter that is determined by the movement distance of the substrate and the liquid repellency of the moving surface.
In a liquid-immersion exposure apparatus, since priority is given to holding the liquid below the final lens, the speed of the stage needs to be kept less than or equal to the limit speed at all times during movement. Therefore, in particular, the movement times of the following movements, including first and second movements, become extremely long compared to those in a non-immersion exposure apparatus. The first movement corresponds to a movement immediately before exposure to a first exposure shot, that is, for example, movement to the first exposure shot after measurement of a positioning standard. The second movement corresponds to a movement immediately after exposure to a final exposure shot, that is, for example, movement for replacing a stage moving below the final lens. Movements for measuring various parameters are also included among the movements. These movements need to be performed through relatively long distances compared to those in the exposure operations. Since the movement times of these long-distance movements become long, the overall time required to process the substrate becomes long, that is, the throughput of the entire apparatus is reduced. Accordingly, in the liquid-immersion exposure apparatus, it is difficult to increase the throughput compared to that in the non-immersion exposure apparatus.
From another aspect, the following problem may arise. That is, a resist that is applied to the substrate tends to react chemically with a liquid, such as pure water. Therefore, depending upon the circumstances, a defect may occur in the resist at the stage of exposure and development. Consequently, an exposure process in which the resist on the substrate has a reduced amount of contact with the liquid is desired.